1. Field of the Invention
The present invention relates to an optical disk and a method of fabricating the optical disk.
2. Discussion of the Related Art
Generally, in a phase-change type optical disk, recorded information is erased in a manner that a local area of a recording layer is heated and melted by irradiating a laser beam onto the area, and then an amorphous mark is made on a crystal matrix through a quick cooling using a disk structure designed to have a quick heat diffusion.
The phase-change type optical disk having the above-described principle is typically composed of a simple four-layer film, i.e., a four-layer film structure in that a lower dielectric layer, a recording layer, an upper dielectric layer, and a reflective layer are stacked on a substrate.
The optical disk may have a structure composed of more than four layers. In this case, any additional layers may be formed among the basic four layers. At this time, the reflective layer is formed at the end.
This structure formed in the above-described order is called a normal stacking structure.
Recently, with the development of a high-density recordable medium, there is a tendency to increase the numerical aperture of an object lens of an optical pickup and to bring the wavelength of a recording optical source into being shorter in an optical recording apparatus.
In this case, as the numerical aperture is increased, the working distance between the object lens and the recording layer of the optical recording medium becomes shorter. Accordingly, the thickness of a transparent substrate that exists in the middle of the optical source and the recording layer should be reduced.
Consequently, the role of the substrate has progressed from its own function for supporting the thin film in the previous normal stacking structure to a thin cover layer for covering the multi-layer thin film that exists on the path through which the laser passes.
Also, in case of increasing the numerical aperture of the lens, a coma aberration due to a tilt that is one of mechanical fluctuations of the disc is greatly increased in proportion to the cube of the numerical aperture. Especially, since the coma aberration becomes greater as the thickness of the substrate positioned on the path through which the incident laser beam passes becomes thicker, the thickness of the substrate should be reduced.
In this case, the stacking order of the thin film is a reversed stacking order in that the reflective layer is first stacked on the contrary to the normal stacking order in that the reflective layer is last stacked.
In such a reversed stacking structure, the reflective layer is first formed on the substrate, other layers are stacked, and then a cover layer is formed over the upper part of the formed structure. Accordingly, the signal characteristics of the optical disk is greatly influenced by the micro structure of the surface of the reflective layer that is the thin film positioned on the lowest part of the structure. Especially, a focus error signal becomes greater in a dynamic driving state of the medium, and this causes a medium noise to be increased.
As a result, a surface roughness of the recording medium that causes the medium noise should be lowered.
In order to lower the surface roughness of the reflective layer, an ion beam sputtering method has been proposed instead of a typical sputtering method. However, this method has the drawbacks in that the time required for the whole process is too long to be suitable for mass production.